In recent years, for environmental protection and fuel economy improvement, a so-called hybrid car including an engine and another power source such as a rotary generator-motor has been developed for practical use. In this type of vehicle, the engine and another power source are used for different running situations.
Idle reduction, for example, is a technology for reducing unnecessary fuel consumption during idling. In the idle reduction technology, an internal combustion engine is stopped when a vehicle stops at a traffic light or in other situations, and the internal combustion engine is restarted by a rotary generator-motor in response to the detection of a driver's intention to start moving, such as stepping on the accelerator or releasing the brake.
The rotary generator-motor mountable in this type of vehicle is mounted in a manner that can transfer torque to the engine. The rotation speed of the rotary generator-motor thus greatly changes by the driver's accelerator pedal operation and the influence of friction of the engine. As a configuration in which an induced voltage of the rotary generator-motor can be controlled independently of the rotation speed of the engine, there has been adopted a field winding type rotary generator-motor capable of control by a field current instead of using a rotor incorporating a permanent magnet.
In general, as a control method for a power generator mounted in a vehicle, there is employed a method of controlling a voltage of a power source such as a battery connected electrically to the power generator to be a predetermined voltage value. In this case, a load torque is generated in an engine connected to the power generator in accordance with a power generation amount. Therefore, when the power generation amount of the power generator varies based on a state of the vehicle, the load torque on the engine also varies in the same manner.
Therefore, when the power generation amount changes abruptly, the generated load torque also increases abruptly. If the abrupt change of the load torque occurs during idling, engine stall may occur, or if the abrupt change of the load torque occurs during driving, its drivability may be affected badly.
Therefore, depending on situations, it is necessary to control the load torque on the engine to be a predetermined value instead of controlling a power source voltage. For instance, there is a related art in which a load torque in power generation is estimated from its field current and rotation speed, and the field current is controlled so that the load torque becomes a predetermined value, to thereby realize the control of the load torque (see, for example, Patent Literature 1). Therefore, when performing the control as described in Patent Literature 1, it is important to correctly detect the field current in order to control the load torque with high accuracy.